Arrangement having tracking if filter

ABSTRACT

In an FM receiver where the center frequency of an IF filter is varied depending on the received signal, an arrangement is described in which the passband of the retuned IF filter is selected so that a distinct resonance region having a relatively steep-edged drop of at least 6 dB exists in the middle of the channel; a less steep or level curve exists in a region essentially symmetrically spaced from the middle of the channel on both sides of the resonance region; and the pass limits lie essentially in symmetry with the middle of the channel.

BACKGROUND OF THE INVENTION

The invention relates to an arrangement of the type defined in thepreamble of claim 1 and to an FM receiver equipped with such anarrangement.

Arrangements of this type are known from DE-OS 3,147,493 and DE-OS3,438,286, EP-A1-75,071 or FR-OS 8,121,986, all of which originate fromthe inventor of the present invention.

In presently employed FM radio receivers, a broadband IF filter isprovided. The center frequency f_(IF) of such a broadband IF filter is10.7 MHz in most standards while the channel bandwidth is about 200 kHz(±100 kHz symmetrically with respect to the center frequency f_(IF)).The passband characteristic is selected in such a manner that itsbandwidth corresponds approximately to the transmission bandwidth. Dueto the wide bandwidth, a relatively large amount of interference andnoise components of the antenna input signal are passed by the prior artIF filter, with the result that relatively high input field strengthsare required to ensure a useful signal (modulation) worthy of beingreceived.

To increase reception sensitivity, it is known from the above-mentionedreferences to use as an IF filter a filter which has a narrow bandwidthrelative to the channel bandwidth. The center frequency of thisnarrow-band IF filter is retuned in dependence on the modulation of thereceived signal. At any arbitrary point in time, selection thus occursexactly where the momentary IF happens to be located. This results inmuch improved suppression of interference and noise components. Thebandwidth of this controlled IF filter, which is also referred to in theliterature as an "in-channel-select" filter or, abbreviated, "ICS"filter, is about 20 kHz as a result of which, however, only themonophonic useful signal component (L+R component of the useful MPXsignal) is passed.

For this reason, it is only possible to operate the ICS filter inparallel with a broadband IF filter in an FM stereo receiver and toeffectively connect the ICS filter with the LF stage of the receiver if,in any case, only monophonic reproduction is possible due to a receivedsignal which is too weak or has too much interference (for example,below 25 μV antenna input voltage).

SUMMARY OF THE INVENTION

In contrast thereto, the invention has the object of achieving in afilter arrangement of the type mentioned above, filtering of the entiresignal, particularly the MPX signal, with adequate sensitivity withoutgreatly increased engineering expenditures.

According to the invention, this is accomplished by the characterizingfeatures of claim 1.

Advantageous features of the filter arrangement according to theinvention are defined in the dependent claims. They also define anadvantageous improvement of an FM stereo receiver equipped with thefilter arrangement.

The invention is initially based on the idea that, in order to providesensitivity, a sharp drop in filtering is required primarily in theimmediate region close to the momentary IF so as to achieve a highsignal/noise ratio with respect to noise components at that point. Incontrast, it is immaterial whether a flat or steeply dropping curvebranch exists in the further frequency curve of the filter passbandcharacteristic.

Instead of, for example, a comb filter structure having discrete filterregions, a coherent passband is provided for the two L-R bands and forthe ARI/RDS auxiliary carrier of the MPX signal which has a relativelylevel frequency curve. A narrow, distinctly higher filter passband witha sharp drop in filtering is superposed on this coherent passband aroundthe center frequency f_(IF). The latter range is the cause of highsensitivity. The bandwidth of this narrow steep-edged filter passband isabout 20 kHz, while the bandwidth of the remaining filter passband whichhas a relatively level frequency curve is about 100 kHz, taking intoaccount the actually contradictory requirements for high sensitivity anda broad passband for the complete MPX signal.

Surprisingly, the advantages of the invention are not only animprovement of the transmission behavior of FM stereo signals. It hasbeen found that the discovered curve for the transmission characteristicpermits the realization of many improvements in the characteristics ofthe ICS process in general. The resonance-like raise in the centerregion results in a significant improvement in the signal to noiseratio. The frequency range of the FM signal which occupy the largestpercentage within the power spectrum are transmitted with maximumamplitude within this raised region. Moreover, the resulting edge shapesin the transition region from the raised region to the flatter regionsfollowing on the sides, makes possible a linear phase response for theuseful signal which contributes to a reduction in non-linear distortion.Additionally, the deviation compatibility is increased.

A particular advantage of the invention is that the discovered frequencyresponse improves the "retunability" of the filters. This is intended tomean the capability of the tunable IF filters to follow the momentarycenter frequency of the modulation. This reduces, in particular, anypossible "constrictions" of the IF at high modulation voltages since therises of the flanks of the passband characteristic as a whole are less.And this also reduces the expenditures for correcting the filter tuningsignal (phase modulation and/or filtering). Moreover, the group delay ofthe useful signals is reduced so that the times to be compensated duringfilter retuning become shorter. As a whole, it must be considered thatthe improvements realized occur in a domain of critical, interfered-withreceiving conditions in which the realized improvements signify aconsiderable increase in "listening pleasure". Experience has shown thatsuddenly starting interferences in an otherwise high-quality (music)reproduction are considered to be particularly annoying.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe drawings, in which:

FIG. 1 shows a passband characteristic of a filter arrangement accordingto the invention;

FIG. 2 is a block circuit diagram of a preferred embodiment of an FMstereo receiver (without preselector and mixer stages;

FIG. 3 shows pass-band characteristics of the various filter stagesprovided in FIG. 2, which, when switched together, result in thepassband characteristic shown in FIG. 1;

FIG. 4 shows the phase response of an LF signal processed by means ofthe filter arrangement according to the invention and demodulated in thereceiver according to FIG. 5; and

FIGS. 5a to 5d show time diagrams of various signal variations within astage for detecting adjacent-channel carriers; and

FIG. 5e is the block circuit diagram of an embodiment of a stage fordetecting adjacent-channel carriers.

DETAILED DESCRIPTION OF THE INVENTION

The passband characteristic of FIG. 1, which will be discussed ingreater detail below, relates to the transmission behavior of the FMstereo receiver according to the invention, an embodiment of which isshown in FIG. 2. The receiver is illustrated without preselector andmixer stages so that a standard IF signal IF₁ at a frequency of 10.7 MHzis present as input signal of the illustrated block circuit diagram.

An essential component of the block circuit according to FIG. 2 is theIF filter arrangement 1 according to the invention, which includes threeseries-arranged filter stages 10, 20 and 30 in a first, upper filterbranch. According to the MPX signal bandwidth of 57 kHz, filter stage10, which determines the relatively level curve section of the filterpassband characteristic according to FIG. 1 and through which pass thehigher frequency components of the MPX signal, has a bandwidth of ±57kHz, that is approximately 100 kHz. In one favorable embodiment, filter10 is not--as would correspond to the solidly drawn transmissionbehavior curve shown in FIG. 3 (also for the other filters)--asingle-circuit filter but a two-terminal bandpass filter. Thetransmission characteristic of this filter, which is more level in thiscase, is marked F₁₀ in FIG. 3 and is shown in dashed lines. Thebandwidths of filter stages 20 and 30 are dimensioned in such a mannerthat the bandwidth of the series circuit of filter stages 10, 20, 30 isapproximately 20 kHz.

The MPX ("MPX"=multiplex) signal under consideration is primarily acurrently used analog FM stereo radio signal. However, it is easilypossible to process a digital FM stereo radio signal with the aid ofthis filter arrangement.

Connecting the three filter stages 10, 20, 30 in series results in themore narrowband frequency curve F₁₀ +F₂₀ +F₃₀, indicated partially indashed lines in FIG. 1. The 3 dB bandwidth of this transmission curve,which coincides with that of the solid-line curve, amounts to about 20%of the transmission bandwidth.

The narrow, steep-edged curve is superposed on the curve of passbandcharacteristic F₁₀ resulting in the lower curve drawn as a solid line inFIG. 1.

Thus, the solid curve shown in FIG. 1 is composed of a central,resonance-like raised region around the middle of the band. At adistance of about 20% of the total bandwidth--seen from the middle ofthe band (that is, in the case of an FM stereo transmission about 20 kHzto the sides of the middle of the band)--where the passbandcharacteristic has already dropped by at least 6 dB, the steeplydropping curve changes to a region which drops less steeply. However,down to the (3 dB) band limit, the drop is still more than 4 dB.

This superposition is effected by circuit means in which the output offirst filter stage 10 is fed to an adder stage 40 in a second filterbranch of filter arrangement 1. The adder stage is also connected to theoutput of the third filter stage 30. Advisably, adder stage 40 is asumming amplifier which amplifies the filtered IF signal to the requiredlevel needed for subsequent demodulation in a demodulator 50. A filterpassband characteristic realized in this manner can also be achieved byother circuit means, for example by means of digital filters, withoutdeviating from the concept of the invention.

Between the illustrated, narrowband curve corresponding to thedashed-line contour and the more broadband solid curve, a switch is madeas a function of the output signal from a switching stage 100 which is ameasure for the characteristics of the received signal. In theillustrated embodiment, this is an adjacent channel interferencedetector as it will be described in greater detail below.

The demodulated signal, designated by LF, at the output of demodulator50 is subjected to a level and possibly phase correction by means of acorrection stage 110 in order to equalize the different level and phasecurves in the two filter branches, that is to say the curves of the twoinput signals of adder stage 40. The corrected LF signal, which includesall components of the MPX signal, is fed to a stereo decoder 60 and to aARI/RDS stage 70.

Stereo decoder 60 generates from the L+R and the L-R components thestereophonic left (L) and right (R) informations which are fed tocorresponding L and R channels respectively. The ARI/RDS stage filtersthe modulated 57 kHz auxiliary carrier of the ARI and/or of the RDSsignal out over a narrow band, demodulates it and (in the case of RDS)decodes the additional ARI and RDS signals and evaluates theseadditional signals. A control stage 80 generates from the LF signal(modulation) at the output of demodulator 50 a positive feedback controlsignal St which is fed to the control inputs of all filter stages. Theadditional ARI signal is understood to mean an Auto Radio Information inthe form of a traffic station and region detection which is modulated inamplitude modulation onto the 57 kHz auxiliary carrier. The termadditional RDS signal is understood to mean a Radio Data Systeminformation which is modulated in the form of a digital signal in 2 PSKmodulation onto the 57 kHz auxiliary carrier in quadrature position inaddition to or independent of the additional ARI signal. In addition totraffic station detection, the RDS information includes a stationidentification, a program type detection as well as alternatefrequencies over which the same program as the tuned-in station isbroadcast (see EBU Doc. Tech. 3244 published by the EuropeanBroadcasting Union).

Additionally, the LF signal is fed to control stage 80, before or aftercorrection. In a manner to be described in greater detail below, controlstage 80 retunes filter stages 10, 20 and 30 to the momentary IF independence on the modulation.

Initially, the bottom curve in FIG. 3 will be considered once more. Thefilter curve designated 2F₁₀ +F₂₀ +F₃₀ in FIG. 3 exhibits, in the regionof the center frequency f_(IF) retuned to the momentary IF, a sharp dropin filtering of at least 6 dB with a width of approximately 20 kHz. Ashas already been mentioned, this sharp drop in filtering results fromthe series connection of filter stages 10, 20, and 30. When the passbandof sum curve F₁₀ +F₂₀ +F₃₀ is superposed on the flat passband of curveF₁₀ in adder stage 4, the signal levels associated with the passbandregions are adjusted in such a manner that the resultant passband ofcurve 2F₁₀ +F₂₀ +F₃₀ is about 10 dB above the passband of curve F₁₀ atits point of resonance. This produces relatively level curve branches ina range which is symmetrically spaced from the middle of the channel(=f_(IF)) by about 38 kHz, adequate for effective noise and interferencesuppression of the L-R band in the MPX signal. The pass limits of theoverall curve (F₁₀ +F₂₀ +F₃₀)/F₁₀ are spaced about 57 kHz from themiddle of the channel so that the auxiliary 57 kHz carrier in the MPXsignal is also filtered effectively.

In the second filter branch of filter arrangement 1 (FIG. 2),an--already mentioned--electronic switch 90 is provided which iscontrolled by a stage 100 for detecting adjacent-channel interference.In the illustrated example, stage 100 is fed from the output of thefirst filter stage and has a switching threshold above which a switchingsignal is supplied to switch 90. This is so because, if the adjacentchannel interference exceeds a limit value, stereo reproduction is nolonger meaningful. At that point, the mentioned superposition of thenarrow, steep-edged curve F₁₀ +F₂₀ +F₃₀ (FIG. 3) on the flat curve F₁₀is prevented and only the mentioned narrow, steep-edged curve becomeseffective for filtering of the IF signal, but only for monophonic signalreproduction. The IF signal resulting in this case at the output ofadder stage 40 exhibits a greater selectivity than in the case whereswitch 90 is closed, as a result of which the detected adjacent-channelinterference is masked out and mono reception free of interference isensured.

The mono/stereo switchover effected by switch 90 may make it possible toomit the presently customary mono/stereo changeover switch in stereodecoder 60.

Stage 100, shown in greater detail in FIG. 5e, includes a seriesconnection of

a first envelope curve demodulator 101;

a high-pass filter 102 having a cutoff frequency of about 60 kHz;

a second envelope curve demodulator 103;

a smoothing stage 104; and

a threshold switching stage 105.

Envelope curve demodulator 101 is supplied with the output signal of thefirst filter stage 10, shown in FIG. 5a, which is amplitude modulated byinterferences between useful and adjacent channel. The stronger theadjacent channel carrier, the greater is this amplitude modulation. Lowfrequency signal components originating from the useful modulation orhigher frequency interference components are removed from thedemodulated interference signal (FIG. 5b) at the output of envelopecurve demodulator 101 by means of a high and/or lowpass filter 102. Inthe subsequent second envelope curve demodulator 103, the envelope curve(FIG. 5c), which is smoothed by means of stage 104 and compared in thethreshold switching stage 105 with a predetermined reference level, isformed via the demodulated, highpass filtered interference signal. Ifthe smoothed envelope curve signal exceeds the reference level,switching stage 105 generates at its output 106 the switching signalshown in FIG. 5d. As is indicated in FIG. 4 by the inputs of stages 2, 3and 4 marked "106", this switching signal is supplied to control stages2, 3 and 4 and also to the control input of switch 90. In the controlstages 2 and 3, the switching signal causes the frequency components ofthe control voltages shown there to be slightly dropped, approximatelyabove 6 kHz. In contrast, in control stage 4 for phase modulators 6, 7,the switching signal causes the frequency components of the controlvoltage generated there to be slightly raised, approximately above 5kHz. These operations for influencing the frequency response arerequired because, due to the changed overall filter characteristic ofthe controlled IF filter stages 10, 20 30, the rate of phase change isincreased which, without the mentioned frequency influencing, would leadto an increased interfering phase modulation in the higher frequencyrange above approximately 5 khz.

In the text which follows, the retuning of filter stages 10, 20 and 30will be discussed in greater detail. For this purpose, three differentphase responses of the LF signal at the output of the demodulator areshown in FIG. 4, the phase response achieved in accordance with theinvention being indicated by a solid line. Due to the retuning of thefilter stages by means of the LF signal or a control signal derived fromit, a phase angle between the IF signal and the control signal occurswhich rises with increasing LF signal frequency. Without the measuresdescribed below, this phase angle would become so large at about 5 kHzthat the positive feedback which is effective up to that point (calledthe "area of positive phase relationship" in FIG. 4) becomes a negativefeedback ("area of negative phase relationship").

Since the phase angle mentioned can be understood to be a phasemodulation of the IF signal, the inventor [of the present invention]already had the idea (DE-OS 3,147,493) to subject the IF signal to anopposite phase modulation before filtering in filter stages 10, 20, 30.As can be seen from FIG. 2, a first retuning control signal for filterstages 20 and 30 is generated in a first control stage 2, and a secondcontrol signal for first filter stage 10 in a second control stage 3, soas to be active as phase modulator. With the aid of this phasemodulation, the phase response with a zero point at 5 kHz, shown indashed lines in FIG. 4, can be shifted in the direction toward higherfrequencies (phase response drawn in dot-dashed lines), so that a zeropoint, i.e., a change from the desired area of positive phaserelationship to the undesirable area of negative phase relationshipoccurs at about 12 kHz; thus, this measure cannot yet be satisfactory.

In the receiver according to FIG. 2, two further phase modulators 6 and7 are provided which are jointly controlled by a third control stage 4.Phase modulator 6 is arranged ahead of a mixer stage 5 which convertsthe incoming standard IF signal IF₁ of 10.7 MHz into a second IF signalIF₂ of, for example, 700 kHz. For this purpose, a local oscillator 8 isused which is followed by the third phase modulator 7. The controlsignals of the control stages 2, 3 and 4 and the frequency responses ofphase modulators 8, 7 and 10 are matched to one another in such a mannerthat the phase response, drawn in a solid line in FIG. 4, of the LFsignal at the output of demodulator 50 is obtained, which remains in theregion of positive phase relationship even at higher LF frequencies. Ithas been found that, on the basis of the good retunability obtained bythe invention, it is possible to omit up to two of the phase modulators,in which case preferably only the phase modulator 6 disposed in themixer stage would remain.

Using the invention, it is possible for the first time and in asurprising manner to receive an MPX signal with considerably increasedsensitivity and to reproduce it in stereo down to about 8 μV antennainput voltage. In this arrangement, filter expenditures are no greaterthan hitherto required for the purely mono solution.

I claim:
 1. In an FM receiver arrangement, an IF filter for filtering areceived FM signal wherein:the IF filter has a relatively narrowbandwidth with respect to a channel bandwidth; the center frequency ofthe IF filter is retuned in dependence on a received signal modulation;and the IF filter having the retuned center frequency provides apassband, wherein within the passband:a) a distinct resonance regionhaving a relatively steep-edged drop of at least 6 dB exists in themiddle of the channel; b) a less steep or level curve exists in a regionessentially symmetrically spaced from the middle of the channel on bothsides of the resonance region; and c) wherein the pass limits of the IFfilter lie essentially in symmetry with the middle of the channel.
 2. Anarrangement according to claim 1, wherein the steep-edged drop amountsessentially to 10 dB and/or the 3 dB bandwidth is only about 20% of thechannel bandwidth.
 3. An arrangement according to claim 1, wherein theregion having the less steep drop and the essentially level curve onboth sides of the middle of the channel begins at a distance of about20% of the channel bandwidth.
 4. An arrangement according to claim 1,wherein the retuned IF filter includes at least two series-connectedfilter stages having respective output signals, with the output signalsof two filter stages being additively linked to form a linked signal andthe linked signal which covers a broader frequency range is fed to asubsequent demodulator stage.
 5. An arrangement according to claim 4,wherein at least one of the at least two filter stages whose outputsignals are additively linked is a bandpass filter having a relativelybroad passband characteristic.
 6. An arrangement according to claim 4,wherein at least one of the at least two filter stages whose outputsignals are additively linked is a single circuit filter having arelatively narrow passband characteristic.
 7. An arrangement accordingto claim 6, wherein two series connected signal circuit filters areprovided which are tuned to the same frequency.
 8. An arrangementaccording to claim 4, wherein the additive linking of the output signalof the first filter stage is effected by controlling a circuit whoseoutput signal is a measure for the reception quality, with the additivelinking being suspended below a limit value of the signal whichconstitutes a measure for the reception quality.
 9. An arrangementaccording to claim 8, wherein the circuit whose output signal is ameasure for the reception quality is formed by an adjacent channeldetection circuit.
 10. An arrangement according to claim 1, wherein theadjacent channel detection circuit includes a series connection composedof the following stages:a first envelope curve demodulator to which isfed the output signal of the first filter stage amplitude modulated byinterferences between useful and adjacent channel; a high and/or lowpass filter which separates the demodulated interference signal at theoutput of the first envelope curve demodulator from high or lowfrequency signal components; a second envelope curve demodulator whichforms the envelope curve by way of the demodulated, highpass filteredinterference signal, with the level of the envelope curve being ameasure for the level of the adjacent channel carrier; and a thresholdswitching stage which compares the possibly smoothed envelope curvesignal with a predetermined reference level and emits a switching signalif the reference level is exceeded.
 11. An FM receiver including anarrangement according to claim 1 to serve as IF filter and a demodulatorconnected to the output of the IF filter; wherein the demodulator isfollowed by a correction stage which is of such a configuration that thedifferences in phase curves and levels resulting from the differentgroup delays and levels of the additively linked filter stages areequalized in the low frequency range.
 12. An FM receiver according toclaim 11, further comprising at least one control stage and second andthird demodulators, whereinthe frequency response of a control voltageof the at least one control stage for the retuned IF filter stages isinfluenced in such a manner that frequency components, for example above6 kHz, are lowered slightly; and each of the frequency responses ofcontrol voltages of the second and third phase modulators are influencedin such a manner that high frequency components beginning at about 5 kHzare raised slightly.
 13. An FM receiver according to claim 10, wherein aseries connection composed of the following stages is provided foradjacent channel detection:a first envelope curve demodulator to whichis fed the output signal of the first filter stage amplitude modulatedby interferences between useful and adjacent channel; a high and/or lowpass filter which separates the demodulated interference signal at theoutput of the first envelope curve demodulator from high or lowfrequency signal components; a second envelope curve demodulator whichforms the envelope curve by way of the demodulated, highpass filteredinterference signal, with the level of the envelope curve being ameasure for the level of the adjacent channel carrier; and a thresholdswitching stage which compares the possibly smoothed envelope curvesignal with a predetermined reference level and emits a switching signalif the reference level is exceeded.
 14. In an FM receiver arrangementhaving an IF filter for filtering a received FM signal in which thecenter frequency of the IF filter, which has a relatively narrowbandwidth with respect to a channel bandwidth, is retuned in dependenceon a received signal modulation, wherein the retuned IF filter includesmeans for selecting the passband of the retuned IF filtercomprising:first means or providing a distinct resonance region having arelatively steep-edged drop of at least 6 dB in the middle of thechannel; second means for providing a less steep or level curve in aregion essentially symmetrically spaced from the middle of the channelon both sides of the resonance region operatively coupled with the firstmeans; and third means for causing the pass limits to lie essentially insymmetry with the middle of the channel operative coupled to the firstand second means.